class MyStack : ICollection, IEnumerable, IEnumerator
    {
        private object[] myArray = new object[0];
        private int currentINDEX = 0, foreachIndex = 0;

        public object Pop()
        {
            object o = myArray[myArray.Length - 1];
            Array.Resize(ref myArray, myArray.Length - 1);
            currentINDEX--;
            return o;

        }

        public void Push(object obj)
        {
            Array.Resize(ref myArray, myArray.Length + 1);
            myArray[currentINDEX++] = obj;
        }

        public int Count
        {
            get { return myArray.Length; }
        }

        public object SyncRoot => throw new NotImplementedException();

        public bool IsSynchronized => throw new NotImplementedException();

        object IEnumerator.Current
        {
            get { return myArray[foreachIndex++]; }
        }

        public void CopyTo(Array array2, int index)
        {
            for (int i = index; i < myArray.Length; i++)
            {
                array2.SetValue(myArray[i], i - index);
            }
        }

        public IEnumerator GetEnumerator()
        {
            return this;
        }

        public bool MoveNext()
        {
            if (myArray.Length - 1 == foreachIndex)
            {
                return false;
            }
            return true;
        }

        public void Reset()
        {
            foreachIndex = 0;
        }
    }